The rough grown surface of a diamond film deposited by the hot filament chemical vapour deposition (CVD) method was polished by thermomechanical polishing on a hot iron plate at 1173 K. The surface composition and structure of the polished diamond film were analysed by Raman spectroscopy, Auger electron spectroscopy (AES), thin film X-ray diffraction (XRD) and secondary ion mass spectrometry (SIMS). The graphitization of the near-surface region of the diamond film by the hot iron plate was observed during thermomechanical polishing. At the diamond film/Si interface, beta-SiC was observed during the early stages of polishing, but after prolonged treatment the graphitization of the diamond film occurred rapidly.